1. Field of the Invention
The present invention relates to a distribution of diffused impurity concentration of a thyristor.
2. Background of the Invention
A power semiconductor device having pn-junction such as a power thyristor provides a ramp formation for processing a semiconductor substrate to increase a surface area of a peripheral part of the semiconductor substrate and then chemically etching a broken layer formed by the processing, in order to protect a pn-junction exposed at the peripheral part of the semiconductor substrate and to give a desired withstand voltage. The ramp formation is performed for purposes of decreasing a field of the surface from a field of the inner part of the semi-conductor substrate by increasing a creeping distance of insulation near the pn-junction exposed at the surface and preventing the decrease of the withstand voltage. The ramp formation is classified into two methods of a negative bevel for providing greater than 90 degree of an angle .theta. between the pn-junction and the ramp surface and a positive bevel for providing less than 90 degree of the angle .theta..
FIG. 1 shows the relation of the angle .theta. and the withstand voltage.
As shown in FIG. 1, the angle .theta. is 45 to 60 degree in the positive bevel and about 2 to 3 degree in the negative bevel whereby the effective area of the semiconductor substrate is maintained and the decrease of the withstand voltage by a punch-through is prevented and the desired dv/dt capability is maintained to obtain high withstand voltage.
In the thyristor having the distribution of diffused impurity concentration shown in FIG. 2, the withstand voltage of the negative bevel is usually lower than that of the positive bevel for about 20 to 30%.
In FIG. 2, the reference (E.sub.1) designates an n type first emitter layer; (B.sub.1) designates a p-first base layer; (B.sub.2) designates an n-second base layer; (E.sub.2) designates a p-second emitter layer; (J.sub.1) designates a pn-junction formed by the second emitter layer (E.sub.2) and the second base layer (B.sub.2); (J.sub.2) designates a pn-junction formed by the second base layer (B.sub.2) and the first base layer (B.sub.1); (J.sub.3) designates a pn-junction formed by the first base layer (B.sub.1) and the first emitter layer (E.sub.1). A width (W.sub.2) of the second emitter layer (E.sub.2) having the distribution of the impurity concentrations lower than that of the first base layer (B.sub.1) near the junction (J.sub.3), is larger than the width (W.sub.1) of the first base layer (B.sub.1).
For example, when the junction (J.sub.2) is in the negative bevel of 2.5 degree and the junction (J.sub.1) is in the positive bevel of 60 degree in the thyristor using the semiconductor substrate having a specific resistance of 35 .OMEGA.-cm, the withstand voltage of the junction (J.sub.2) is 1,000 V and the withstand voltage of the junction (J.sub.1) is 1,600 V. The withstand voltage of the junction (J.sub.2) is remarkable lower than that of the junction (J.sub.1). That is, the withstand voltage of the thyristor is 1,000 V.
In order to overcome these disadvantages, the structure having the distribution of diffused impurity concentrations shown in FIG. 3 is used for a power thyristor. The first base layer (B.sub.1) and the second emitter layer (E.sub.2) are formed in a structure comprising a region A having a small ramp of the impurity concentrations and a region B having a large ramp of the impurity concentrations by a double diffusion method.
When the junction (J.sub.2) is reverse-biased, a depletion layer is expanded to the first base layer (B.sub.1) and a field intensity is weakened to increase the withstand voltage and a punch-through in the first base layer (B.sub.1) is prevented to provide a desired dv/dt capability. This structure is mainly used.
When a semiconductor substrate having a specific resistance of 35 .OMEGA.-cm used and the junction (J.sub.2) is in the negative bevel and the junction (J.sub.1) is in the positive bevel in the structure of the thyristor, the withstand voltage of the junction (J.sub.2) is 1,350 V and the withstand voltage of the junction (J.sub.1) is 1,600 V. Although the withstand voltage of the junction (J.sub.1) is not increased the withstand voltage of the thyristor is 1,350 V. The increase of the withstand voltage is remarkably high in comparison with the thyristor having the distribution of diffused impurity concentrations shown in FIG. 2.
On the other hand, it is well known that a forward voltage drop of a thyristor is increased depending upon an increase of a width of the second base layer (B.sub.2).
In a case of a pnpn structure, the width and the specific resistance of the second base layer (B.sub.2) are decided depending upon the withstand voltage of the thyristor. The width of the second base layer (B.sub.2) is larger and the specific resistance is larger depending upon the increase of the withstand voltage of the thyristor. Thus, the forward voltage drop is higher depending upon the increase of the withstand voltage.
A power consumption of the semiconductor device per area is limited depending upon a heat resistance of the semiconductor device. The increase of the forward voltage drop limits a permissible current capacity. In order to increase the capacity, the forward voltage drop should be lowered.
A turn-on current density for deciding the permissible current capacity of the thyristor is in a range of about 100 to 300 A/cm.sup.2. When the current having the turn-on current density of about 100 to 300 A/cm.sup.2 is fed to a thyristor of a pnpn structure having an n type first emitter layer (E.sub.1), it is well known that the electron density doped from the first emitter layer (E.sub.1) into the first base layer (B.sub.1) is in a range of 10.sup.17 to 10.sup.18 /cm.sup.3. Therefore, when the current density is greater than 100 A/cm.sup.2, the pnpn type thyristor has substantially the same structure as the pn structure. This is also considered in the second emitter layer (E.sub.2) at the junction (J.sub.1). When the current density is greater than 100 A/cm.sup.2, the first base layer (B.sub.1) and the second emitter layer (E.sub.2) which have the impurity concentration of 10.sup.17 to 10.sup.18 /cm.sup.3 have the same structure as that of the second base layer (B.sub.2). When the junction (J.sub.2) is in the negative bevel and the junction (J.sub.1) is in the positive bevel in the thyristor having the distribution of diffused impurity concentrations shown in FIG. 3, the withstand voltage is increased but the forward voltage drop is disadvantageously increased.